1. Field of the Invention
The present invention relates generally to distance measuring devices and, more particularly, to an apparatus for measuring radial distances relative to the surface of a cylinder, e.g. the transfer cylinder of an offset printing press.
2. Discussion of the Prior Art
In an offset printing press, an image is formed on a plate cylinder, and is transferred to a blanket or transfer cylinder which applies the image to a sheet or web that is passed between the transfer cylinder and an impression cylinder.
The plate, transfer and impression cylinders used in the printing press are each provided with bearers at each axial end thereof and a central circumferential section that is recessed slightly in the radial direction relative to the bearers. A plate is supported on the central circumferential section of the plate cylinder, while a blanket and possibly additional layers of packing material are supported on the central circumferential sections of the plate and transfer rollers.
In order to achieve high quality printing, it is desirable to provide a particular amount of squeeze at the nip between the plate and transfer cylinders, as well as at the nip between the transfer and impression cylinders. For example, in a typical offset printing operation a squeeze of 0.003 inches is desired between the plate and transfer cylinders, and a squeeze of between 0.005-0.007 inches is sought between the transfer cylinder and the web or sheet that is passed between the transfer cylinder and the impression cylinder.
When initially setting up the press for operation, the thicknesses of the plate, blanket, and web are known. However, in order to obtain the desired squeeze pressures, it is necessary to determine the overall height of the blanket and packing material on the transfer cylinder so that the thickness of the packing material may be adjusted to provide the desired squeeze between the blanket and plate.
It is conventional to use an elongated, V-shaped bar for carrying out such measurements on the transfer cylinder. The bar is inverted and placed on the surface of the transfer cylinder so that an end of the bar abuts one of the bearers to provide a point from which a radial measurement may be made with a conventional measuring device.
Numerous problems arise during use of this conventional arrangement. For example, because the bar rests in edge contact on the cylinder surface, the bar is unstable. This problem is typically most evident when a high spot exists on the transfer roller, causing the bar to become wobbly such that the point from which a measurement is to be made cannot be isolated.
The instability of the V-shaped bar also introduces a problem in that it is difficult to accurately align the center line of the bar with the longitudinal axis of the cylinder. The inability to provide this alignment reduces the accuracy of measurements taken off of the bar, and adversely effects the repeatability of measurements.